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Name ____________________________
Earthquakes
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Date ___________________
Class ____________
Section Summary
Forces in Earth’s Crust
Guide for Reading
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How does stress in the crust change Earth’s surface?
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Where are faults usually found, and why do they form?
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What land features result from the forces of plate movement?
The movement of Earth’s plates creates enormous forces that squeeze or pull
the rock in the crust. A force that acts on rock to change its shape or volume
is stress. Stress adds energy to the rock. The energy is stored in the rock until
it changes shape or breaks.
Three different kinds of stress can occur in the crust—tension,
compression, and shearing. Tension, compression, and shearing work over
millions of years to change the shape and volume of rock. Tension pulls on
the crust, stretching rock so that it becomes thinner in the middle.
Compression squeezes rock until it folds or breaks. Shearing pushes a mass
of rock in two opposite directions.
When enough stress builds up in rock, the rock breaks, creating a fault.
A fault is a break in the rock of the crust where rock surfaces slip past each
other. Most faults occur along plate boundaries, where the forces of plate
motion push or pull the crust so much that the crust breaks. There are
three main types of faults: normal faults, reverse faults, and strike-slip
faults.
Tension causes a normal fault. In a normal fault, the fault is at an angle,
and one block of rock lies above the fault while the other block lies below the
fault. The block of rock that lies above is called the hanging wall. The rock
that lies below is called the footwall. Compression causes reverse faults. A
reverse fault has the same structure as a normal fault, but the blocks move
in the opposite direction. Shearing creates strike-slip faults. In a strike-slip
fault, the rocks on either side of the fault slip past each sideways, with little
up or down motion.
Over millions of years, the forces of plate movement can change a flat
plain into landforms such as anticlines and synclines, folded mountains,
fault-block mountains, and plateaus. A fold in rock that bends upward into
an arch is an anticline. A fold in rock that bends downward to form a valley
is a syncline. Anticlines and synclines are found on many parts of the Earth’s
surface where compression forces have folded the crust. The collision of two
plates can cause compression and folding of the crust over a wide area.
Where two normal faults cut through a block of rock, fault movements may
push up a fault-block mountain. The forces that raise mountains can also
uplift, or raise plateaus. A plateau is a large area of flat land elevated high
above sea level.
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Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Review and Reinforce
Forces in Earth’s Crust
Understanding Main Ideas
Use the diagrams below to answer items 1–3.
Diagram A
Diagram B
Diagram C
1. Diagram A
b. Stress Force: ____________________
c. Movement Along Fault: ________________________________________________
2. Diagram B
a. Type of Fault: ____________________
b. Stress Force: ____________________
c. Movement Along Fault: _________________________________________________
3. Diagram C
a. Type of Fault: ____________________
b. Stress Force: ____________________
c. Movement Along Fault: ________________________________________________
Building Vocabulary
Write a definition for each of these words. Use the back of this sheet if you need
more space.
4. shearing ________________________________________________________________
5. hanging wall ____________________________________________________________
6. syncline ________________________________________________________________
7. footwall ________________________________________________________________
8. stress __________________________________________________________________
9. anticline ________________________________________________________________
10. plateau _________________________________________________________________
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Earthquakes
a. Type of Fault: ____________________
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Section Summary
Earthquakes and Seismic Waves
Guide for Reading
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How does the energy of an earthquake travel through Earth?
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What are the scales used to measure the strength of an earthquake?
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How do scientists locate the epicenter of an earthquake?
An earthquake is the shaking and trembling that results from the movement
of rock beneath Earth’s surface. The point beneath Earth’s surface where
rock under stress breaks to cause an earthquake is called the focus. The point
on the surface directly above the focus is called the epicenter. During an
earthquake, vibrations called seismic waves move out from the focus in all
directions. Seismic waves carry the energy of an earthquake away from the
focus, through Earth’s interior, and across the surface.
There are three categories of seismic waves: P waves, S waves, and
surface waves. P waves compress and expand the ground like an accordion.
S waves vibrate from side to side and up and down. When P waves and
S waves reach the surface, some become surface waves. Surface waves move
more slowly than P waves and S waves.
Three commonly used methods of measuring earthquakes are the
Mercalli scale, the Richter scale, and the moment magnitude scale. The
Mercalli scale was developed to rate earthquakes according to the level of
damage at a given place. An earthquake’s magnitude is a number that
geologists assign to an earthquake based on the earthquake’s strength. The
Richter scale is a rating of an earthquake’s magnitude based on the size of
the earthquake’s seismic waves. The seismic waves are measured by a
seismograph. A seismograph is an instrument that records and measures
seismic waves. Geologists today often use the moment magnitude scale, a
rating system that estimates the total energy released by an earthquake. An
earthquake’s magnitude tells geologists how much energy was released by
the earthquake. The effects of an earthquake increase with magnitude.
Geologists use seismic waves to locate an earthquake’s epicenter.
When an earthquake strikes, P waves arrive at a seismograph first and
S waves next. The farther away the epicenter is, the greater the difference
between the two arrival times. This time difference tells scientists how far
from the seismograph the epicenter is. The scientists then use the
information from three different seismograph stations to plot circles on a
map. Each circle shows the distance from one seismograph station to all the
points where the epicenter could be located. The single point where the three
circles intersect is the location of the earthquake’s epicenter.
© Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Review and Reinforce
Earthquakes and Seismic Waves
Understanding Main Ideas
Answer the following questions in the spaces provided.
1. What are seismic waves?
_________________________________________________________________________
_________________________________________________________________________
2. In what order do the three types of seismic waves arrive at a seismograph?
_________________________________________________________________________
_________________________________________________________________________
3. Which type of seismic wave produces the most severe ground movements?
_________________________________________________________________________
4. Describe the moment magnitude scale, and explain why it is useful in
measuring earthquakes.
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
5. How do geologists locate the epicenter of an earthquake?
_________________________________________________________________________
_________________________________________________________________________
_________________________________________________________________________
Building Vocabulary
Match each term with its definition by writing the letter of the correct definition in
the right column on the line beside the term in the left column.
____
6. focus
____
7. epicenter
____
8. surface waves
____
9. seismograph
____ 10. magnitude
a. records ground movements caused by
seismic waves as they move through the
Earth
b. slowest seismic waves that produce the
most severe ground movements
c. the point beneath Earth’s surface at
which rock under stress breaks and
triggers an earthquake
d. a measurement of earthquake strength
e. the point on the surface directly above
the point at which an earthquake occurs
© Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Section Summary
Monitoring Earthquakes
Guide for Reading
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How do seismographs work?
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How do geologists monitor faults?
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How are seismographic data used?
Many societies have used technology to try to determine when and where
earthquakes have occurred. During the early 1900s, scientists developed
seismographs that were much more sensitive and accurate than any earlier
devices. A simple seismograph can consist of a heavy weight attached to a
frame by a spring or wire. A pen connected to the weight rests its point on a
drum that can rotate. As the drum rotates slowly, the pen draws a straight
line on paper that is wrapped tightly around the drum. Seismic waves cause
the seismograph’s drum to vibrate. But the suspended weight with the pen
attached moves very little. Therefore, the pen stays in place and records
the drum’s vibrations. The pattern of lines, called a seismogram, is the
record of an earthquake’s seismic waves produced by a seismograph.
To monitor faults, geologists have developed instruments to measure
changes in elevation, tilting of the land surface, and ground movements
along faults. A tiltmeter measures tilting or raising of the ground. A creep
meter uses a wire stretched across a fault to measure horizontal movement
of the ground. A laser-ranging device uses a laser beam to detect horizontal
fault movements. A network of Earth-orbiting satellites called GPS helps
scientists monitor changes in elevation as well as horizontal movement
along faults.
Seismographs and fault-monitoring devices provide data used to map
faults and detect changes along faults. Geologists are also trying to use
these data to develop a method of predicting earthquakes. Geologists use
the data from seismic waves to map faults, which are often hidden by a thick
layer of rock or soil. This practice helps geologists determine the earthquake
risk for an area. Geologists use fault-monitoring devices to study the types
of movement that occur along faults. Friction is the force that opposes the
motion of one surface as it moves across another surface. Where friction
along a fault is low, the rocks on both sides of the fault slide by each other
without much sticking. Stress does not build up, and large earthquakes are
unlikely. Where friction is high, the rocks lock together. Stress builds up until
an earthquake occurs. Even with data from many sources, geologists can’t
predict when and where a quake will strike.
© Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Review and Reinforce
Monitoring Earthquakes
Understanding Main Ideas
Answer the following questions on a separate sheet of paper.
1. How might monitoring faults help geologists predict an earthquake?
2. What two factors help geologists determine earthquake risk?
Seattle
Boston
Minneapolis
Chicago
San
Francisco
Los Angeles
Salt
Lake
City
Cleveland
Washington, D.C.
St.
Louis
Denver
New York
Philadelphia
Phoenix
Atlanta
Charleston
Houston
New
Orleans
KEY
Risk of Damage
None
Minor
Moderate
Major
3. a. Name three cities shown on the map above that have a major risk of
earthquake damage.
b. Name three cities that have a moderate risk of earthquake damage.
c. Name three cities that have a minor risk of earthquake damage.
Building Vocabulary
Answer each item below. Write your answers on a separate sheet of paper.
4. Name four instruments that are used to detect movement along faults.
5. Briefly describe how each instrument works.
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Miami
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Section Summary
Earthquake Safety
Guide for Reading
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How do geologists determine earthquake risk?
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What kinds of damage does an earthquake cause?
■
What can be done to increase earthquake safety and reduce
earthquake damage?
© Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.
Earthquakes
Geologists can determine earthquake risk by locating where faults are
active and where past earthquakes have occurred. In the United States, the
risk is highest along the Pacific Coast in the states of California, Washington,
and Alaska. The eastern United States generally has a low risk of
earthquakes because this region lies far from plate boundaries.
Causes of earthquake damage include shaking, liquefaction,
aftershocks, and tsunamis. The shaking produced by seismic waves can
trigger landslides or avalanches. The types of rock and soil determine where
and how much the ground shakes. Liquefaction occurs when an earthquake’s
violent shaking suddenly turns loose, soft soil into liquid mud. As the ground
gives way, buildings sink and pull apart. Sometimes, buildings weakened by
an earthquake collapse during an aftershock. An aftershock is an earthquake
that occurs after a large earthquake in the same area.
When an earthquake jolts the ocean floor, plate movement causes the
ocean floor to rise slightly and push water out of its way. The water
displaced by the earthquake may form a large wave called a tsunami. A
tsunami spreads out from an earthquake’s epicenter and speeds across the
ocean. The height of the wave is low in the open ocean, but the wave grows
into a mountain of water as the tsunami approaches shallow water.
The main danger from earthquake strikes is from falling objects and
flying glass. The best way to protect yourself is to drop, cover, and hold. To
prepare for an earthquake, store in a convenient location an earthquake kit
containing canned food, water, and first aid supplies.
Most earthquake-related deaths and injuries result from damage to
buildings or other structures. To reduce earthquake damage, new
buildings must be made stronger and more flexible. Older buildings may
be modified to withstand stronger quakes. The way in which a building is
constructed determines whether it can withstand an earthquake. A baseisolated building is designed to reduce the amount of energy that reaches
the building during an earthquake. During a quake, the building moves
gently back and forth without any violent shaking.
Earthquakes can cause fire and flooding when gas pipes and water mains
break. Flexible joints and automatic shut-off valves can be installed to
prevent breaking and to cut off gas and water flow.
Name ____________________________
Earthquakes
■
Date ___________________
Class ____________
Review and Reinforce
Earthquake Safety
Understanding Main Ideas
Answer the following questions on a separate sheet of paper.
1. What types of damage do earthquakes cause?
2. How do ground conditions affect earthquake damage to buildings?
3. Why are aftershocks dangerous to buildings after a large earthquake?
4. Why do tsunamis grow into larger waves as they approach land?
5. If an earthquake strikes while you are indoors, what should you do?
6. Why should people living in earthquake regions prepare
emergency kits?
Earthquakes
7. What kind of building design is shown in this
figure? Explain how the design helps reduce
earthquake damage.
Foundation
Building Vocabulary
Write a word to complete each sentence correctly.
8. The water displaced by a strong earthquake on the ocean floor forms
large waves called ____________________.
9. ____________________ occurs when an earthquake’s violent shaking
suddenly turns loose, soft soil into liquid mud.
10. A(n) ____________________ is an earthquake that occurs after a large
earthquake centered in the same area.
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